Earlier studies have provided molecular methods by which rabbit MHC antigens can be conveniently typed. The goal of the present study is to use animals with known MHC types in order to determine the effect of these antigens on immune responsiveness. In addition, basic studies of MHC gene structure have continued emphasizing sequence analysis of DQalpha class II genes from the rabbit. Several studies are underway in which cellular responses to certain proteins are being tested using MHC typed rabbit cells. These studies center about development of immunity to structural and regulatory proteins of the human retrovirus HIV-1. Antibody and cellular immune responses to the envelope protein gp160 and to the regulatory protein nef are being studied. Immunity to these proteins has been elicited by injection of either protein or peptide and responses to the protein and peptide are being tested. Rabbits having cellular and humoral responses that neutralize virus will be used for challenge with live HIV-1 in an attempt to determine whether these immunization procedures provide protection against virus infection.